1. Field of the Invention
The present invention relates to a fixed focal length lens system, and more particularly to a fixed focal length lens system consisting of as few lens components as possible and suitable for use in a camera based on a solid-state image-sensing device.
2. Description of the Prior Art
In recent years, miniaturization and cost reduction have been eagerly sought in cameras (for example, video cameras and television cameras) of the type that produces an image of an object by receiving light rays incoming through a taking lens system by means of a solid-state image-sensing device (for example, a charge-coupled device (CCD)) composed of an array of photosensitive components. As a natural consequence of this trend, miniaturization and cost reduction have been sought also in taking lens systems for use in such cameras.
Miniaturization and cost reduction in taking lens systems can most effectively be achieved by reducing the number of the constituent lens components. In fact, up to date, various taking lens systems have been proposed that consist of as few lens components as possible. For example, Japanese Laid-open Patent Applications Nos. H1-307712, H2-73210, H4-349418, and others propose fixed focal length taking lens systems in which miniaturization and cost reduction are achieved by reducing the number of the constituent lens components. Moreover, Japanese Laid-open Patent Application No. H5-341185 proposes an objective optical system consisting of two lens components for use in an endoscope.
On the other hand, in recent years, much attention has been given to the fact that a combination of a diffractive optical component and a refractive optical component effectively corrects chromatic, spherical, and other aberrations. In fact, up to date, various optical systems have been proposed that employ such combinations of optical elements. For example, Japanese Laid-open Patent Application No. H6-324262 proposes a taking lens system based on this principle, and Japanese Laid-open Patent Application No. H6-242373 proposes an objective lens based on the same principle for use in an optical disk system.
In a camera based on a solid-state image-sensing device as mentioned above, a condensing microlens is provided on the incident-surface side of each photosensitive component in the image-sensing device to enhance the light reception efficiency of the photosensitive component. To maximize the light condensing ability of such a condensing microlens, it is desirable that light rays entering the microlens be parallel to the optical axis of the microlens (that is, even off-axial light rays entering the microlens be perpendicular to the incident surface of the corresponding photosensitive component).
The light condensing ability of the microlens is maximized by placing the exit pupil of the taking lens system substantially at infinity (that is, by designing the taking lens system to be substantially telecentric toward the image side), because, then, off-axial light rays entering the microlens are substantially perpendicular to the incident surface of the corresponding photosensitive component. By contrast, if the exit pupil of the taking lens system is placed closer to the image plane, off-axial light rays entering those microlenses which are situated at the edges of the image formed on the solid-state image-sensing device form oblique angles with the incident surfaces of the corresponding photosensitive components, with the result that the light condensing ability of the microlenses weakens at the edges of the image. This leads to uneven brightness between the central and peripheral portions of the image. An optical system that is designed, for the above-stated reason, to be substantially telecentric toward the image side is proposed, for example, in Japanese Laid-open Patent Application No. H1-128025 and others in the form of a fixed focal length taking lens system for use in a video camera.
However, the fixed focal length taking lens system designed to be substantially telecentric as proposed in Japanese Laid-open Patent Application No. H1-128025 and others requires four or more lens elements, and therefore it is difficult to achieve further miniaturization and cost reduction in it. The fixed focal length taking lens systems as proposed in Japanese Laid-open Patent Applications Nos. H1-307712, H2-73210, H4-349418, and others are not designed to be substantially telecentric, and therefore the microlenses in solid-state image-sensing device used with them exhibit poor light condensing ability, causing uneven brightness between the central and peripheral portions of the image as mentioned above. In addition, these taking lens systems do not offer satisfactory optical performance in terms of correction of aberrations, in particular chromatic aberration. The objective optical system proposed in Japanese Laid-open Patent Application No. H5-341185 does not offer satisfactory optical performance when employed as a taking lens system in a camera based on a solid-state image-sensing device. The taking lens system proposed in Japanese Laid-open Patent Application No. H6-324262 requires as many as five or more lens components, and the objective lens proposed in Japanese Laid-open Patent Application No. H6-242373 does not offer satisfactory optical performance when employed as a taking lens system in a camera based on a solid-state image-sensing device.